Light emitting diodes (LED) are used as light sources for a large-size illumination unit such as that used for backlight of TV sets. Slimness and evenness of illuminance are required for such an illumination unit.
FIG. 1 shows an example of a structure of an illumination unit using LEDs used for backlight. The illumination unit includes LED light sources 200, optical elements 100, a reflecting plate 300 and a diffusion plate 400. Each of the LED light sources 200 are covered by one of the optical elements 100. The optical elements 100, each covering each of the LED light sources 200, are arranged at regular intervals on the reflecting plate 300. The diffusion plate 400 is provided above the reflecting plate 300 and the optical elements 100.
Lights emitted by the LED light sources 200 are reflected by the optical elements 100 toward the rear (in the direction toward the reflecting plate 300). Then the lights reflected toward the rear are further reflected by the reflecting plate 300, diffused by the diffusion plate 400 and reach a plane to be illuminated. Thus, evenness of illuminance on the plane to be illuminated is realized. Various types of illumination units using reflecting plates and diffusion plates such as those described above have been developed (Patent Documents 1 to 3). The planar light emitting device described in Patent Document 1 is not of a structure in which optical elements contain light sources, and therefore a sufficient slimness of the illumination device cannot be realized. On the other hand, in the optical element of lateral light-emitting type described in Patent Document 2, in which a light source chip is imbedded in the center of a lens and in the lens of lateral light-emitting type described in Patent Document 3, which is provided with a concave portion for arranging a light source in the light receiving side, a part of lights is directed directly from the sides of the lens to a plane to be illuminated without being reflected by the lens. As a result, unevenness of illuminance is generated on the plane to be illuminated in such a way that outlines of the lenses appear at the positions on the plane, which correspond to the lenses.
Thus, an optical element for an illumination unit and an illumination unit which are capable of realizing both slimness and a high evenness of illuminance on a plane to be illuminated, have not been developed.